1. Field of the Invention
The invention concerns a superconductive material and the method of its manufacture and, more particularly, the making of devices with thin films of superconductive materials having high critical temperatures.
More precisely, the invention concerns a structure which enables precise control over certain constituents of the superconductive material. For example, the invention enables precise control over the oxygen or fluorine content of a superconductive material of the type Y--Ba--Cu--O--F.
2. Description of the Prior Art
Since the discovery, in 1986, of the superconductivity, at 30 K, of the compound La.sub.2-x Ba.sub.x CuO.sub.4-y, by BEDNORZ and MULLER (Z. Phys. B.64. 189.1986), the achieved critical temperature of superconductive oxides has been constantly rising. This is the result of an unprecedented degree of mobilization of the international scientific community. Thus, the new compounds of the system Y--Ba--Cu--O now have a critical temperature of the order of 95 K. It must be noted that, with the compound studied by BEDNORZ and MULLER, only the substitution of lanthanum has enabled the critical temperature to be raised to 95 K. The manipulation and replacement of the other metal cations have had no effect to date.
More recently, the replacement of a part of the oxygen by fluorine has enabled the critical temperature in the system Y--Ba--Cu--O--F, to be further raised to about 150 K (cf. article by S. R. OVSHINKSY et al in "Physical Review Letters", 15th June 1987, p. 2579, and also article by MENG XIAN-REN et al in "Solid State Communications", vol. 64, No. 3, 1987, p. 325.
However, the observed phenomenon of loss of resistivity does not appear to be reproducible and, for reasons as yet unexplained, this phenomenon disappears after a few cycles of temperature between the ambient temperature and that of liquid nitrogen (see article by MENG XIAN-REN et al referred to above).
Furthermore, the exact anionic composition leading to the appearance of the high-temperature superconductivity of oxyfluoride is difficult to obtain. This phenomenon has already been observed in the oxides of the system Y--Ba--Cu--O as described in the French patent application No. 87 09463 of 3rd July 1987. Thus, it is observed that the superconductive properties are highly dependent on the oxygen and fluorine content of the material.
The method according to the invention can be used, firstly, to obtain stable, thin layers of superconductive oxyfluoride Y--Ba--Cu--O--F and, secondly, to adjust their oxygen and fluorine composition with precision.
More generally, an object of the invention is a device made of superconductive material, the structure of which enables the content of each of the anionic elements of the superconductive material to be fixed with precision, as well as a method which, by means of a structure of this type, can be used to achieve this precision.